Disposable nonwoven absorbent products have widespread acceptance for infant, young child and incontinent adult care applications. Typical disposable nonwoven absorbent articles include diapers, training pants, adult incontinent pads and briefs, feminine sanitary napkins or pads and tampons. Disposable nonwoven absorbent articles such as those mentioned function to receive and contain urine and other body fluids that the wearer secretes. These items are worn against or in close proximity to the skin of the wearer.
Typical disposable nonwoven absorbent articles consist of a fluid—impervious film back sheet, a porous fluid permeable nonwoven top sheet and an absorbent core sandwiched between the top and back sheets. These articles are usually bonded using hot melt adhesives. In addition to this basic construction, these absorbent articles usually have many other features to either improve the body fluid containment function or to enhance the comfort level for the wearer. For example, infant diapers contain elastic leg cuffs attached to the top sheet for enhanced fluid containment.
Since disposable nonwoven absorbent articles are widely used for body fluid containment function, it is desirable to know whether the article is wet and thus requires replacement. Monitoring of wetness by visual inspection can be time consuming and unpleasant. It is therefore invaluable to incorporate a function to signal wetness into a disposable nonwoven absorbent article such as a diaper.
Approaches using coating stripes of wetness indicator adhesives, or wetness indicators to signal wetness by way of color change have been described in Mroz et al., U.S. Pat. No. 4,231,370. This article discloses an improved absorbent product having a wetness indicator disposed between a translucent cover member and an absorbent member. According to the disclosure, the wetness indictor is applied in the form of a stripe to a portion of the inwardly facing surface of a back sheet of a disposable diaper. Such a wetness indictor contains a pH—change/color—change type of colorant dispersed in a water-based adhesive latexes of styrene/2-ethylhexylacrylate copolymer, ethylene/vinyl acetate copolymer, or polyvinyl acetate. The indicator adheres to the back sheet and dries to a flexible coating that is yellow in color. When insulted by body secretions such as urine, the indicator changes from yellow to blue, signaling the presence of moisture. To obtain a suitable pH, sufficient acid buffering means such as phosphoric acid must be added to the latexes. Phosphoric acid is a harsh acid, which could raise safety concerns. Another disadvantage of the Mroz et al latex-based composition and any similar water or solvent-based products is that the means of water or solvent removal has to be provided during manufacturing.
Colon et al. U.S. Pat. Nos. 4,681,576, 4,743,238 and 4,895,567 disclose hot melt wetness indicator adhesives that change color upon insult with urine or water. These adhesives are based on a water-soluble polyvinyl pyrrolidone polymer, or a water soluble vinyl pyrrolidone—vinyl acetate copolymer, or an ethylene-acrylic acid copolymer in combination with a fatty acid and a wetness indicating dye. The composition can contain a variety of other ingredients such as water-soluble waxes, glycerol esters, ethylene—vinyl acetate copolymers and hydrogenated oils, etc.
Zimmel et al., U.S. Pat. No. 5,035,691 discloses a hot melt wetness indicator composition based on an adduct which is prepared by reacting ethylene-acrylic acid copolymer with polyethylene oxide under monobutyl tin (IV) oxide catalyst. The composition contains 0.03 to 0.5 wt % acid-base indicator as the active ingredient to signal the presence of moisture.
Raykovitz, U.S. Pat. No. 5,342,861 discloses a composition similar to that of Zimmel et al. in that the composition comprises a wetness indicating agent such as a pH indicator, a graft copolymer prepared by reacting a vinyl polymer with low molecular weight polyethylene oxide and a compatible tackifier.
The prior art compositions herein mentioned above have several deficiencies. The hot melt wetness indicator composition disclosed in Colon et al, for example, exhibits poor thermal stability. Thus, when heated at elevated temperatures between 250-300° F., which is typically encountered during hot melt application, the adhesives can severely degrade as manifested by char, skin formulation and color darkening. Most of the components in Colon's composition are incompatible with each other, and therefore, the composition can suffer from phase separation during application at the typical hot melt adhesive coating conditions. Other deficiencies are the poor environmental stability and poor bleed-through or wash-out resistance that typical polyvinylpyrrolidone homo- or co-polymer based formulations suffer after the indicator is applied to a typical polymer film substrate. The coated indicator tends to change slowly and prematurely from yellow to green and finally to blue from exposure to atmospheric moisture during storage. This aspect is particularly important since finished nonwoven adsorbent products can be stored for a few months before they reach consumer's hands. A premature color change during storage will render the product useless. An additional deficiency is the poor intensity of color change of the indicator when insulted resulting in the color change being barely visible through translucent substrates. The compositions taught by Zimmel et al. and Raykovitz, on the other hand, necessitate harsh conditions to carry out chemical grafting of low molecular weight hydrophilic PEG to another relatively high molecular weight hydrophobic polymer. Their grafting reactions require either an organotin catalyst (Zimmel) or a peroxide initiator (Raykovitz). Problems can arise from product safety concerns with residual organotin compounds and peroxides. Since the hydrophilic PEG is typically incompatible with the vinyl polymer used for preparation of the graft copolymer, the unreacted reactants can pose compatibility problems for the final wetness indicator composition.
In view of the deficiencies of the prior art products, needs exist for a new wetness indicating adhesive composition that is compatible, that is thermally and environmentally stable, that has intense color change and good wash-out resistance, that can withstand multiple insults during use, and that is easy to manufacture and apply.
All of the prior art disclosures involve the use of various indicators that change color under visible light conditions either in response to changes in pH when contacted with a liquid such as urine or when contacted by a liquid per se. However, it would be advantageous to be able to check for wetness in a dark environment, for example while the wearer is sleeping, without having to wake the person. The present invention discloses the use of wetness indicating adhesive compositions containing a fluorescing agent that does not fluoresce when dry, but instead is activated and becomes visible to the human eye in the dark and/or under a UV light only when the article becomes wet. This feature is not disclosed or suggested in any of the prior art listed.